Transformer Fault Categories

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Jocelin Morrison
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1 Transformer Fault Categories 1. Winding and terminal faults 2. Sustained or uncleared external faults 3. Abnormal operating conditions such as overload, overvoltage and overfluxing 4. Core faults 1 (1)

2 Transformer Protection (1) Transformer Connections Overcurrent Protection Directional Protection of Parallel Transformers Partial Differential Protection of Parallel Transformers Earth Faults on Transformer Windings Unrestricted Earth Fault Protection Restricted Earth Fault Protection Biased Differential Protection of 2 and 3 Winding Transformers 2 (2)

3 Transformer Protection (2) Combined Differential and Restricted Earth Fault Protection Protection of Auto-Transformers Inter-Turn Faults and Buchholz Protection Overfluxing Protection Overload Protection Transformer Feeder Protection 3 (3)

4 Transformer Connections 4 (4)

5 Transformer Protection (3) I A2 Ø V E P A1 5 (5)

6 Transformer Protection (4) I A2 a2 V E P E S A1 a1 6 (6)

7 Transformer Protection (5) I P A2 a2 I S V E P E S A1 a1 7 (7)

8 Transformer Connections A a a2 C2 C C1 B1 A2 A1 B2 B c2 c c1 a1 b1 b b2 A B A2 B2 A1 B1 a1 b1 a2 b2 a b Clock face numbers refer to position of low voltage phase - neutral vector with respect to high voltage phase - neutral vector. Line connections made to highest numbered winding terminal available. C C2 C1 c1 c2 c Line phase designation is same as winding. 8 (8)

9 Transformer Vector Groups Group 1 0 Group Group 3 30 Group 4 30 Phase displacement Phase displacement Lag phase displacement Lead phase displacement Yy0 Dd0 Zd0 Yy6 Dd6 Dz6 Yd1 Dy1 Yz1 Yd11 Dy11 Yz11 9 (9)

10 Transformer Connections Clock Face numbers refer to position of low voltage phase-neutral vector with respect to high voltage phase neutral vector Line connections made to highest numbered winding terminal available Line phase designation is same as winding A Phase Windings B Phase Windings C Phase Windings Example 1 : Dy 11 Transformer A2 B2 C2 High Voltage Windings A1 B1 C1 a1 b1 c1 Low Voltage Windings a2 b2 c2 10 Question : How to connect windings? (10)

11 Dy (11)

12 Dy Draw Phase-Neutral Voltage Vectors A Line Designation a 30 b C B c 12 (12)

13 Dy Draw Delta Connection A a b C B c 13 (13)

14 Dy Draw A Phase Windings A A2 a a2 a1 b A 1 C B c 14 (14)

15 Dy Complete Connections (a) A a a2 C1 A2 C C 2 B 1 B 2 A 1 B c 2 c c 1 a1 b1 b2 b 15 (15)

16 Dy Complete Connections (b) A A 2 A 1 a1 a2 a B B 2 B 1 b1 b2 b C C2 C1 c 1 c 2 c 16 (16)

17 11kV Distribution Transformers Typical Fuse Ratings Transformer rating Fuse kva Full load current (A) Rated current (A) Operating time at 3 x rating(s) (17)

18 Traditional Small Transformer Protection Package 3.3k V 200/ N 50 N 1500/5 1MVA 3.3/0.44kV 51 N /5 18 (18)

19 Traditional Medium Transformer Protection Package 11kV /5 5MVA 11/3.3kV 51 N /5 19 (19) 3.3kV

20 Overcurrent Protection 20 (20)

21 Transformer Overcurrent Protection Requirements Fast operation for primary short circuits Discrimination with downstream protections Operation within transformer withstand Non-operation for short or long term overloads Non-operation for magnetising inrush 21 (21)

22 Use of Instantaneous Overcurrent Protection Source HV LV set to x through fault level 22 (22)

23 Transient Overreach Concerns relay response to offset waveforms (DC transient) Definition I 1 - I 2 I 2 x I 1 I 2 D.C. (23) I 1 = Steady state rms pick up current I 2 = Fully offset rms pickup current

24 Instantaneous High Set Overcurrent Relay Applied to a Transformer HV 2 Time HV 1 HV1 HV2 LV L V 24 (24) I I F(HV F(LV) 1.2I F(LV ) ) Current

25 2-1-1 Distribution (1) I 3Ø I 3Ø I 3Ø 0.866I 3Ø 25 (25)

26 2-1-1 Distribution (2) HV relay LV relay 0.4 sec I 3Ø I 3Ø 26 (26)

27 Parallel Transformers Directional Relays (1) Grid supply Feeders (27)

28 Parallel Transformers Directional Relays (2) 51 Grid supply 51 Bus Section Feeders (28)

29 Parallel Transformers Partial Differential Scheme Grid supply P1 P2 S1 S S2 P2 S1 P Advantage : Reduced (29) number of grading stages

30 Earth Fault Protection 30 (30)

31 Transformer Earth Faults 3 p.u. turns 1 p.u. turns I P x PR Protective Relay R I F ResistorlimitsE/Fcurrenttofullloadvalues For afaultat : Faultcurrent. F.L. Thus, primary current, P x. F.L.. F.L. 31 Effectiveturnsratio 3 : If C.T.ratio(onprimary side)is basedon full loadcurrentof transforme r,thenc.t.secondary circuit (31) 2 3

32 Overcurrent Relay Sensitivity to Earth Faults (1) I f as multiple of I F.L. 51 Overcurrent Relay I F Star Side Delta Side Overcurrent Relay Setting > I F.L. p.u.. 32 (32)

33 Overcurrent Relay Sensitivity to Earth Faults (2) I f as multiple of I F.L. 51 Overcurrent Relay I F Star Side Delta Side p.u.. 33 (33)

34 Overcurrent Relay Sensitivity to Earth Faults (3) I f as multiple of I F.L. 10 I F 9 I F 8 7 I P I N I P I N p.u.. 34 (34)

35 Earth Fault on Transformer Winding I = 2 3 I Differential Relay Setting = I S For relay operation, I > I S 2 e.g. If I S = 20%, then > 20% for operation 3 i.e. > 59% Thus 59% of winding is not protected 35 Differential Relay Setting % of Star Winding Protected 10% 58% 20% 41% 30% 28% 40% 17% 50% 7% (35)

36 Unrestricted Earthfault Protection (1) 51N Provides back-up protection for system Time delay required for co-ordination 36 (36)

37 Unrestricted Earthfault Protection (2) 51N 51N Can provide better sensitivity (C.T. ratio not related to full load current) (Improved effective setting) Provides back up protection for transformer and system 37 (37)

38 Star Winding REF Protected Zone RE F Relay only operates for earthfaults within protected zone. Uses high impedance principle. 38 Stability level : usually maximum through fault level of transformer (38)

39 Restricted E/F Protection Low Voltage Windings (1) A B C N LV restricted E/F protection trips both HV and LV breaker Recommended setting : 10% rated 39 (39)

40 Restricted E/F Protection Low Voltage Windings (2) A B C N LV restricted E/F protection trips both HV and LV breaker Recommended setting : 10% rated 40 (40)

41 Delta Winding Restricted Earth Fault Source Protected zone REF 41 Delta winding cannot supply zero sequence current to system Stability : Consider max LV fault level Recommended setting : less than 30% minimum earth fault level (41)

42 High Impedance Principle Protected Circuit Z R C R C Z M T R L I I F T R L M V S SR S R L R T R L 42 Voltage Across Relay Circuit V S = I F (R CT + 2R L ) Stabilising resistor R ST limits spill current to I S (relay setting) V S R ST = - R R where R R = relay burden I S CT knee point V KP = 2V S = 2I F (R CT + 2R L ) (42)

43 Non-Linear Resistors (Metrosils) During internal faults the high impedance relay circuit constitutes an excessive burden to the CT s. A very high voltage develops across the relay circuit and the CT s. Causes damage to insulation of CT, secondary winding and relay. Magnitude of peak voltage V P is given by an approximate formula (based on experimental results) Where V F setting V P = 2 2V K (V F - V K ) = Swgr. Fault Rating in amps x Z of relay CT ratio Metrosil required if V P > 3kV 43 (43)

44 Non-Linear Resistors (Metrosils) I O P V M V S R ST R R Metrosil Characteristic V = C I Suitable values of C & chosen based on : Max secondary current under fault conditions Relay setting voltage 44 (44)

45 REF Protection Example 80MV A 1MVA (5%) 11000V 415V 1600/1 R CT = 4.9 Calculate : 1) Setting voltage (V S ) 2) Value of stabilising resistor required 3) Effective setting 1600/1 R CT = 4.8 R S MCAG14 I S = 0.1 Amp 4) Peak voltage developed by CT s for internal fault 2 Core 7/0.67mm (7.41 /km) 100m Long 45 (45)

46 Solution (1) Earth fault calculation :- Using 80MVA base p.u. Source impedance = 1 p.u. Transformer impedance = 0.05 x 80 = 4 p.u. 1 1 P.U I 1 I 2 I 0 Total impedance = 14 I 1 = 1 = p.u. 14 Base current = 80 x x 415 = Amps I F = 3 x x = Amps (primary) = 14.9 Amps (secondary) 46 Sequence Diagram (46)

47 Solution (2) (1) Setting voltage V S = I F (R CT + 2 RL ) Assuming earth CT saturates, R CT = 4.8 ohms 2 RL = 2 x 100 x 7.41 x 10-3 = ohms Setting voltage = 14.9 ( ) (2) Stabilising Resistor (R S ) = 93.6 Volts R S = V S - 1 I S I S 2 Where I S = relay current setting R S = = 836 ohms (47)

48 Solution (3) 1.6 Weber/m2 (Tesla) (multiply by Kv to obtain RMS secondary volts) Kv Ki Line & Neutral CT Earth CT AT/mm (multiply by Ki to obtain total exciting current in Amps) (48)

49 Solution (4) (3) Effective setting I P = CT ratio x (I S + I MAG ) Line & Neutral CTs Flux density at 93.6V = 93.6 = Tesla 158 From graph, mag. Force at Tesla = AT/mm Mag. Current = x = Amps Earth CT Flux density at 93.6V = 93.6 = Tesla 236 From graph, mag. Force at Tesla = AT/mm Mag. Current = x = Amps Thus, effective setting = 1600 x (0.1 + [(4 x ) ]) Effective setting = 198 Amps Transformer full load current = 1391 Amps Effective setting = 198 x 100% = 14.2% x rated (49)

50 Solution (5) (4) Peak voltage = 2 2V K (V F - V K ) V F = 14.9 x V S = 14.9 x 936 = Volts I S For Earth CT, V K = 1.4 x 236 = 330 Volts (from graph) V PEAK = 2 2 x 330 x ( ) = 6kV Thus, Metrosil voltage limiter will be required. 50 (50)

51 Parallel Transformers T 1 N A B C Bus Section T Volt Switchboard (51)

52 T1 Parallel Transformers CT in Earth N A B C 51N Bus Section Open T 2 51N 415 Volt Switchboar d 52 (52)

53 T1 Parallel Transformers CT in Earth and Neutral N A B C 51N Bus Section Open T 2 51N 415 Volt Switchboar d 53 (53)

54 Parallel Transformers Residual Connections T1 N A B C F2 Bus section Will maloperate if bus section is open for fault at F1 T2 F1 415 volt switchboard No maloperation for fault at F2 (but setting must be greater than load neutral current) 54 (54)

55 Traditional Large Transformer Protection Package 33K V 200/ MVA 33/11K V 600/5 51 N /5 5/5A 55 (55)

56 Differential Protection 56 (56)

57 Differential Protection Overall differential protection may be justified for larger transformers (generally > 5MVA). Provides fast operation on any winding Measuring principle : Based on the same circulating current principle as the restricted earth fault protection However, it employs the biasing technique, to maintain stability for heavy through fault current Biasing allows mismatch between CT outputs. It is essential for transformers with tap changing facility. Another important requirement of transformer differential protection is immunity to magnetising in rush current. 57 (57)

58 Biased Differential Scheme Differential Current I 1 BIAS BIAS I 2 I 1 - I 2 OPERATE OPERATE I 1 - I 2 RESTRAIN I 1 + I 2 2 Mean Through Current 58 (58)

59 Differential Protection HV PROTECTED ZONE LV R Correct application of differential protection requires CT ratio and winding connections to match those of transformer. CT secondary circuit should be a replica of primary system. Consider : 59 (1) Difference in current magnitude (2) Phase shift (3) Zero sequence currents (59)

60 Differential Connections P1 P2 A2 A1 a1 a2 P2 P1 A B C 60 (60)

61 Use of Interposing CT P1 P2 A2 A1 a1 a2 P2 P1 S1 S2 S2 S1 S2 S1 P1 P2 R R R 61 Interposing CT provides : Vector correction Ratio correction Zero sequence compensation (61)

62 15MVA 150/5 66kV / 11kV 800/5 P1 P2 A2 A1 a1 a2 P2 S1 S2 Differential Protection S2 S1 P1 Dy1 62 Given above: Need to consider - (1) Winding full load current (2) Effect of tap changer (if any) (3) C.T. polarities Assuming no tap changer Full load currents:- However, require 11kV C.T. s to be connected in 66kV: 131 Amp = 4.37 Amps secondary 11kV: 787 Amp = 4.92 Amps secondary Thus, secondary current = 3 x 4.92 = 8.52A RATIO CORRECTION IS REQUIRED (62)

63 150/5 800/5 P1 P2 A2 A1 a1 a2 P2 P1 S1 S2 S2 S1 Differential Protection 4.37A S1 S2 P1 P2 4.92A R R R (2.56) (5) 63 It is usual to connect 11kV C.T. s in and utilise a / interposing C.T. (this method reduces lead VA burden on the line C.T. s) Current from 66kV side = 4.37 Amp Thus, current required from winding of int. C.T. = 4.37 Amp Current input to winding of int. C.T. = 4.92 Amp Required int C.T. ratio = 4.92 / 4.37 = 4.92 / May also be expressed as : 5 / 2.56 (63)

64 Effect of Tap Changer e.g. Assume 66kV +5%, -15% Interposing C.T. ratio should be based on mid tap position Mid Tap (-5%) Primary current (15 MVA) Secondary current = 62.7 kv = 138 Amp = 4.6 Amp Interposing C.T. ratio required = 4.92 / May also be expressed as : 5 / 2.7 ( / ) = 4.92 / 2.66 Compared with 5 / 2.56 based on nominal voltage 64 (64)

65 Connections Check Arbitrary Current Distribution P1 P2 A2 A1 a1 a2 P2 P1 S1 S2 S2 S1 S2 S1 P1 P2 R R R 65 (65)

66 Connections Check Add Delta Winding Current P1 P2 A2 A1 a1 a2 P2 P1 S1 S2 S2 S1 S2 S1 P1 P2 R R R 66 (66)

67 Connections Check Complete Primary Distribution P1 P2 A2 A1 a1 a2 P2 P1 S1 S2 S2 S1 S2 S1 P1 P2 R R R 67 (67)

68 Connections Check Complete Secondary Distribution P1 P2 A2 A1 a1 a2 P2 P1 S1 S2 S2 S1 S2 S1 P1 P2 R R R 68 (68)

69 In-Zone Earthing Transformer P1 P2 A1 a1 A2 a2 P2 P1 S2 S1 S2 S1 T2 T1 P1 P2 69 (69)

70 P1 P2 A1 In-Zone Earthing Transformer Alternative (1) a1 A2 a2 P2 P1 S2 S1 S2 S1 P1 P2 70 (70)

71 In-Zone Earthing Transformer Alternative (2) 300/1 EARTHING TRANSF. 900/1 71 (71) TO DIFFERENTIAL RELAY

72 In-Zone Earthing Transformer Alternative (3) 300/1 EARTHING TRANSF. 300/1 1/ (72) TO DIFFERENTIAL RELAY

73 Combined Differential and Restricted Earth Fault Protection A2 A1 a1 a2 P1 P2 S1 S2 P1 P2 S1 S2 REF P2 P1 S1 To differential relay S2 73 (73)

74 Combined Differential and Earth Fault Protection Using Summation Auxiliary Current Transformer Restricted earth fault relay Bias windings Differential relay operating windings 74 (74)

75 PHASE a Z T 1 X PHASE b PHASE c A A A N B G G G T 1 BEF F T 2 T 3 J J J T 4 Rb 2 Rb 2 Rb 2 H H H Ro Ro Ro 75 OVERALL DIFFERENTIAL RELAY (75)

76 Three Winding Transformer 300/5 63MV 25MV A A 132KV 11KV 1600/5 50MV A 33KV 1000/ All interposing C.T. ratio s refer to common MVA base (63MVA) (76)

77 Traditional Use of Interposing CT Dy1(-30 ) Yd11(+30 ) R R R Interposing CT provides : Vector correction Ratio correction Zero sequence compensation 77 (77)

78 Integral Vectorial and Ratio Compensation Power transformer Virtual interposing CT Numeric Relay Differential element Ratio correction Vectorial correction Virtual interposing CT 78 (78)

79 Transformer Magnetising Characteristic Twice Normal Flux Normal Flux 79 Normal No Load Current No Load Current at Twice Normal Flux (79)

80 Magnetising Inrush Current Steady State + m V I m - m 80 (80)

81 Magnetising Inrush Current Switch on at Voltage Zero - No residual flux 2 m Im V 81 (81)

82 Transformer Differential Protection Effect of Magnetising Current Appears on one side of transformer only Seen as fault by differential relay Normal steady state magnetising current is less than relay setting Transient magnetising inrush could cause relay to operate 82 (82)

83 Transformer Differential Protection Effect of Magnetising Inrush SOLUTION 1 : TIME DELAY Allows magnetising current to die away before relay can operate Slow operation for genuine transformer faults 83 (83)

84 Transformer Differential Protection Effect of Magnetising Inrush SOLUTION 2 : 2ND (and 5TH) HARMONIC RESTRAINT Makes relay immune to magnetising inrush Slower operation may result for genuine transformer faults if CT saturation occurs Used in MiCOM P63x 84 (84)

85 Transformer Differential Protection Effect of Magnetising Inrush SOLUTION 3 : GAP MEASUREMENT TECHNIQUE Inhibits relay operation during magnetising inrush Operate speed for genuine transformer faults unaffected by significant CT saturation Used in MBCH & KBCH relays 85 (85)

86 Typical Magnetising Inrush Waveforms A B C 86 (86)

87 Detection of Typical Magnetising Inrush (50Hz) Bias Differential Threshold Differential comparator T1 = 5ms T2 = 22ms Trip 87 (87)

88 Restraint for Transformer Magnetising Inrush Bias Differential Threshold Differential comparator T1 = 5ms T2 = 22ms Trip Differential input Comparator output T1 T2 Trip Reset 88 (88)

89 Operation for Transformer Faults Bias Differential Threshold Differential comparator T1 = 5ms T2 = 22ms Trip Differential input Comparator output T1 T2 Trip Reset 89 (89)

90 Protection of Auto-Transformer by High Impedance Differential Relays (a) Earth Fault Scheme A B C 87 High impedance relay 90 (90)

91 Protection of Auto-Transformer by High Impedance Differential Relays (b) Phase and Earth Fault Scheme A B C a b c n 91 (91)

92 Inter-Turn Fault Protection 92 (92)

93 Inter-Turn Fault E CT Shorted turn Load Nominal turns ratio Fault turns ratio Current ratio - 11,000 / ,000 / 1-1 / 11,000 Requires Buchholz relay 93 (93)

94 Interturn Fault Current / Number of Turns Short Circuited Fault current in short circuited turns 8 Fault current (multiples of rated current) Primary input current 6 4 Primary current (multiples of rated current) Turn short-circuited (percentage of winding) 94 (94)

95 Buchholz Relay Installation 5 x internal pipe diameter (minimum) 3 x internal pipe diameter (minimum) Conservator Oil conservator 3 minimum Transformer 95 (95)

96 Buchholz Relay Petcock Alarm bucket Counter balance weight Mercury switch Oil level To oil conservator From transformer Trip bucket Aperture adjuster Drain plug Deflector plate 96 (96)

97 Overfluxing Protection 97 (97)

98 Overfluxing Generator transformers Grid transformers Usually only a problem during run-up or shut down, but can be caused by loss of load / load shedding etc. 98 Flux V f Effects of overfluxing : Increase in magnetising current Increase in winding temperature Increase in noise and vibration Overheating of laminations and metal parts (caused by stray flux) Protective relay responds to V/f ratio Stage 1 - lower A.V.R. Stage 2 - Trip field (98)

99 Overfluxing Basic Theory V = kf 2 m m CAUSES Low frequency I e High voltage Geomagnetic disturbances EFFECTS Tripping of differential element (Transient overfluxing) Damage to transformers (Prolonged overfluxing) 99 (99)

100 V/Hz Overfluxing Protection V f K Trip and alarm outputs for clearing prolonged overfluxing Alarm : Definite time characteristic to initiate corrective action Trip : IDMT or DT characteristic to clear overfluxing condition Settings Pick-up 1.5 to 3.0 i.e. 110V x 1.05 = Hz DT setting range 0.1 to 60 seconds 100 (100)

101 V/Hz Characteristics Enables co-ordination with plant withstand characteristics t = x K (M - 1) 2 Operating time (s) K = 63 K = 40 K = 20 K = 5 K = 1 M = V Hz Setting 101 (101)

102 Overfluxing Relay Ex G VT AVR R L 102 (102)

103 Application of Overfluxing Relay Circuit breaker position repeat relay VAA relay DC RL1-1 Lower AVR DC Inhibit AVR raise Alarm RL2-1 RL2-2 Alarm Generator field circuit breaker trip coil 103 (103)

104 Thermal Overload Protection 104 (104)

105 Effect of Overload on Transformer Insulation Life Relative rate of using life With ambient of 20 C. Hot spot rise of 78 C is design normal. A further rise of 6 C doubles rate of using life Hot spot temp C (105)

106 Overheating Protection TD setting I load Trip Alarm Top oil of power transformer I load On Off Fan control On Pump control Heater Off Temp. indication Thermal replica Temperature sensing resistor Local Remote 106 (106)

107 Overload Protection Overcurrent protection designed for fault condition Thermal replica provides better protection for overload Current based Flexible characteristics Single or dual time constant Reset facility Time Non-volatile Current 107 (107)

108 Thermal Overload Oil Filled Transformers Trip time (s) Single characteristic: = 120 mins Dual characteristic Current (multiple of thermal setting) Single characteristic: = 5 mins 108 (108)

109 Thermal Trip Time TripTime I IREF ln 2 I I REF 2 K P TRIP where I I REF P TRIP K = heating time constant = actual current measured by relay = continuous current rating of protected plant = previous thermal state = trip threshold = multiplier (for actual temperature) 109 (109)

110 Transformer Feeders 110 (110)

111 Higher - voltage busbar Protection of Parallel Transformer Feeders Z OC OC Z FTS FTS REF REF DP DP Bh WT WT Bh REF REF SBEF 2 stage SBEF 2 stage DOC OC OC DOC 111 Load Load Lower - voltage busbar (111)

112 Protection of Transformer Feeders CTs CTs CTs HV LV CTs TRIP Feeder Differential Protection PILOTS Feeder Differential Protection Transformer Differential Protection UNSTABILISE TRIP TRIP 112 (112)

113 Transformer Feeders FEEDE R PW PILOT S P W For use where no breaker separates the transformer from the feeder. 113 Transformer inrush current must be considered. Inrush is a transient condition which may occur at the instant of transformer energisation. Mag. Inrush current is not a fault condition Protection must remain stable. MCTH provides a blocking signal in the presence of inrush current and allows protection to be used on transformer feeders. (113)

114 114 ( ) Dy11 P2 P1 II III I S1 S2 MVTW 02 MFAC 14 PILOTS MBCI MBCI MCTH MCTH C B A ii iii i P1 P2 S2 S1 C B A S1 Transformer Feeder Protection

115 P541/ P542 - Protection of Transformer Feeders Power transformer P540 Scheme Ratio correction Vectorial correction Virtual interposing CT Virtual interposing CT 115 (115)

Transformer Protection

Transformer Protection Nature of transformer faults TXs, being static, totally enclosed and oil immersed develop faults only rarely but consequences large. Three main classes of faults. 1) Faults in Auxiliary